In recent years the need for rapid, high volume liquid/liquid separation in the oil industry has increased. Most of the secondary and tertiary recovery methods developed in the United States and Canada utilize large quantities of water, resulting in produced mixtures of oil and water which are difficult to separate. Even the major oil producing countries in the Middle East are now beginning to produce ever increasing volumes of brine with their oil, increasing the demand for high volume desalting equipment.
New developments and processes in other industries are also requiring liquid/liquid separation equipment. A good example in the mining industry is the solvent extraction processes. The chemical industry is utilizing similar liquid ion exchange processes. All of these processes require elaborate liquid/liquid mixing and/or separating facilities.
The use of high voltage electric fields to force the separation of oil field emulsions is a well known and accepted practice. These fields greatly speed the coalescence and separation of immiscible liquids, over conventional heater treaters and settlers using mechanical aids to coalescence. However, considerable retention time is still necessary, and large vessels are required if large volumes of emulsion are to be processed in a short time.
"Retention Time" is that period required for a first fluid dispersed in a second fluid to settle into a single body from which it can be removed. Many things will affect retention time. A large factor is the size of the drops formed by the dispersed fluid. Considering gravity to be the usual external force applied to the dispersed drops, if the diameter of these drops are doubled, their falling velocity through the fluid in which the drops are dispersed will be increased 10 times under Stokes Law. An electric field is a tool which has been used to increase the size of the dispersed drops by forcing separated drops to join each other, or coalesce. The increase in the falling velocity of the coalesced drops will enable the size of the retaining vessel required for retention time to be greatly reduced.
Another tool in the separation art is the centrifuge. The centrifuge generates a force usually expressed in "G's," G being the normal force of gravity. It is not unusual to develop the centrifugal force in a centrifuge to 1,000 G's in value. Many attempts have been made to apply centrifugal force on the heavier fluid of a fluid mixture to separate it from the less dense fluids of the mixture.
The centrifuge has had varying degrees of success in separating gas from liquids, gas from solids and liquids from solids. However, experience with liquid/liquid separation has been frustrating. The oil-water mixtures of the oil field have met bewildering limitations in centrifuge separation.
One of the reasons for the liquid/liquid separation limitations with the centrifuge is that the difference in densities of the liquids is not great enough. This is a vague statement, of course. But a more important problem can be stated more specifically.
When the dispersed phase is very fine, that is, the drops of dispersed liquid are down to the 2-5 micron range, the centrifugal force necessary to move the drops will develop shear forces between the liquids which will prevent further enlargement of the drop size. The mixture is therefore stabilized by centrifugal force at a dispersion which limits separation.
To breach the limitation on dispersed drop growth by liquid shear, it is evident that dispersed drops must be enlarged to a size by another force which will enable the drops to receive only enough centrifugal force to move the drops in separation without developing the shear force which will refragment the drops.